Backfiring Options

[BfloRandy]

Happy Spring, aka fix your car so you can drive it all summer!

Having an issue tuning my Weber 40's.

The car does run, and idles quite well, sounds smooth and even. If I rev beyond 3k, I get backfires. Starts fine too. Ordinarily I would assume it's a simple matter of resetting and resyncing.

Passenger side carb is as it always has been, fine. Mix adjustment screws work normally, idle adjustment also. I see the squirt from the accelerator jets.

Driver side, not so much. It's getting gas. For some reason, the mix screws have little to no effect. This is on BOTH cylinders. The only time I notice any change in the engine's sound is if I back them way way out, then I get backfiring and carb throat sneezing. Otherwise, I can have them set just about anywhere, including fully seated, and there is no difference. I've pulled the carb off, (three times now) torn it down and blown everything out a couple of times already. I did see some blockage in the idle jets and mains before I blew them out.

I also noticed that I didn't see any squirt from the accelerator jets. I tore it down again, and fiddled with the pump (new last spring) and reassembled it. Accel jets now appear to work normally.

It doesn't at all help that my kids decided to reorganize a ton of things, and I now can't find my Weber manual. (IMG:style_emoticons/default/chair.gif)
Can anyone help? Thanks!

[Elliot Cannon]

I overhauled both my weber 44idf's this year. After I put them back together it still ran like crap. I pulled the idle jets (again) and they were clogged. Check the idle jets again. It's easy after you've done it about 50 times. (IMG:style_emoticons/default/biggrin.gif) These help a lot. https://www.cbperformance.com/product-p/7557.htm

[Justinp71]

At around 3k rpm's you are transitioning to your main circuit. Are you certain you have the correct main fuel and air jets?

Adjusting the idle mixture screws should have little affect. Also you might want to look into your ignition system, I have had that be the culprit in the past. Check for good wires, plugs connections, points, etc...

Goodluck!

[ThePaintedMan]

On the offending carb that the idle mixture screws do not respond on, does the car idle lower if you manually pull the linkage closed more? If so, the carb is not operating fully on the idle circuit, and therefore the mixture screws won't have much/any effect. But first -?

When is the last time you adjusted the valve lash? Do this first.
What is you idle speed?
Have you synchronized the carbs at idle with the linkage disconnected? Do this next.

If so, then try and synch the carbs again at 2500-3000 rpms, or at least get a reading. I am leaning towards one carb being ahead of the other.

Is the backfiring coming from one carb only? If so, then you need to verify your floats are working and set properly.

If not, then suspect low fuel volume across the entire system. Do you have a fuel pressure gauge installed? What kind of fuel pump?

[BfloRandy]

Ok, time for an update, and some replies.

Elliot, I have pulled the idle jets a few times, they're still pretty clear.

Justin, yes, at 3k I'm moving to the mains. I replaced the jets last spring to conform to what is listed in the Weber manual. Now using 55 idles and 115 mains. Adjusting the idle mixture screws is necessary to set the carbs to best lean idle, isn't it? Cylinder 1 does nothing. Cylinder 2 gets rough if I open it WAY up, like 2 or 3 full turns. New plugs are going in in a few minutes, will check wires (though they're new last spring also) and timing, etc.

George, so many questions! (IMG:style_emoticons/default/biggrin.gif)
There is one prog port exposed below the throttle valve on Cylinder 2. Always been that way, hasn't had too much effect in the past. Idle screws worked normally. Yes, idle does drop a little when rotating the linkage bar manually, also always been this way, and the throttle valves appear to close fully. (Looked at them while it was off the car)
Valves were adjusted late last summer.
Idle speed was at about 1200, I have it at 950 now, hope to keep it there!
Just synched the carbs, disconnected. Backfiring is actually worse now.
Can't tell which carb is backfiring. Just reset float on driver's side, as it seems to be the issue. Will check passenger side in a bit when I'm pulling plugs.
I get a nice stream of gas when I disconnect the line (only 3.5 psi) using a mechanical rotary pump I bought from CB Performance. I don't believe that is the issue. It's 10 times the crappy Facet one it replaced!

[injunmort]

what are you running for ignition? are you sure it is not breaking down by not advancing? sure you are timed correctly?

[BfloRandy]

what are you running for ignition? are you sure it is not breaking down by not advancing? sure you are timed correctly?

Bosch blue coil, 009 dizzy. Just checked the timing. 27 BTDC (2.0L)

[BfloRandy]

Just replaced plugs 1 & 2. 1 looked normal, 2 was black with carbon. I expected that, because of that prog port being exposed as I mentioned earlier. Checked timing and dwell. Timing is good, dwell is a tad off, but at 41 degrees so not horrific. Plug wires all seem like new.

I get no backfires or carb sneezes when at idle. I only get backfiring when I rev over 3k. Going to go pull the passenger side carb and clean it out. I've been convinced that the issue is on the other side because of the mixture screws, but, it wouldn't be the first time I was an idiot! Won't be the last. (IMG:style_emoticons/default/confused24.gif)



ok, just pulled and put back carb #2. cleaned idle jets, mains were clean. Adjusted float. Backfiring is better, but still present. Also now getting some stumbling on accel and popping and crackling on decel. Vacuum leak? This is getting really frustrating!

[BfloRandy]

what are you running for ignition? are you sure it is not breaking down by not advancing? sure you are timed correctly?

Bosch blue coil, 009 dizzy. Just checked the timing. 27 BTDC (2.0L)

Not sure what you mean by "breaking down by not advancing" ?

[BfloRandy]

ok, new update:

Just pulled off both carbs again, cleaned them out, again, also pulled intake runners and checked gaskets there.

One issue has apparently been resolved: the air/fuel mix screws are working normally on both carbs, all 4 barrels. Yay! (IMG:style_emoticons/default/piratenanner.gif)

Still backfiring once revved to 3k+.
I checked all of the wiring to the coil, plug wires are all seated properly.
Timing is good, set at 27 BTDC at 3200. Dwell is 41 degrees, so could be better, will reset with new points.
Idle is a little high however, about 1300. Idle speed screws are backed out all the way. I should have that around 1k, I think?

The car runs nicely when idling. No popping, sneezing or backfiring. I only get the backfires when revving over 3k. (Main jets are clear) When I release the throttle, I get a little popping as it drops back down to idle.

Now I'm turning towards a valve adjustment (just did one a few hundred miles ago last fall, perhaps I did it poorly?) and check for exhaust leaks.
Any more ideas?

[rhodyguy]

To be clear. The idle speed screws are backed completely out, the arms are against the stops and the idle is at 1.3k?

[BfloRandy]

To be clear. The idle speed screws are backed completely out, the arms are against the stops and the idle is at 1.3k?

Yes. A (mal)function of timing? I can easily knock it down to 1k, but then I'm running with retarded timing. I've always had to fight with it to keep the idle down, but never like this! Usually no more than 1200 on a bad day, 1k when I get it just SO.
I'm also wondering if there is an issue with my coil or dizzy causing me to not get a good enough spark.

[BfloRandy]

Idle now at 1200, Both carbs are adjusting. Still backfiring.

[rhcb914]

To me sounds like you have an ignition problem.

Last year towards the end of the season mine started to run like crap. Skipping at idle, really crappy running at mid-range and occasional backfiring through the carbs. I pulled the idle and main jets. Replaced the fuel filter. Still ran just as crappy.

Threw in a new set of plugs (old ones were 8 years old), bingo. Ran perfect.

I find many "carb" problems, aren't carb related.

[injunmort]

when was the condensor last changed? it could that simple. they are cheap, used to part of any tune-up kit and they do fail.

[BfloRandy]

Thanks for the ideas, guys. I've changed plugs 1 & 2 (got dark). Going after points tomorrow morning. They and condenser were new last spring, but yeah, cheap so will swap out condenser and rotor too.

I just find it really odd that while the car idles fine (ok, a little high but no other issues) it backfires only when revved. Another 914 gremlin I suppose... Maybe after another 20 years I'll get this down pat.

Thanks again!

[McLovin]

Thanks for the ideas, guys. I've changed plugs 1 & 2 (got dark). Going after points tomorrow morning. They and condenser were new last spring, but yeah, cheap so will swap out condenser and rotor too.

I just find it really odd that while the car idles fine (ok, a little high but no other issues) it backfires only when revved. Another 914 gremlin I suppose... Maybe after another 20 years I'll get this down pat.

Thanks again!

Cars will backfire when they have a vacuum leak because the mixture gets too lean and detonates at the wrong time. Cars will pull more vacuum at higher RPM, so sometimes vacuum leaks don't show up unless you're at higher RPM. Have you checked for vacuum leaks? Try this. Get some starter fluid, and spray around your intake system. Be careful not to start a fire, but look for a sudden subtle increase in RPM when you spray the starter fluid...that's a clue that you're spraying close to the location of the vacuum leak.

If you did this already, I apologize...just trying to help.

[Justinp71]

I had a problem with my electronic points once where it idled fine and then when I tried to put power down it backfired. Ended up being the points got loose inside the distributor, man that one was tough to find. Probably a mute point here since I think he has mechanical points... but figured I'd share.

[BfloRandy]

Most recent update:

Just replaced points, rotor and condenser.
Dwell at 45 degrees, timing set at 27 BTDC at 3200 rpm.
Idling at 1400. Idle adjust screws are backed off of the stops. Only way I can get it lower is to manually pull up on the droplink (hard) on driver's side carb. This gets it down to 11-1200.
Still getting popping (not full blown backfiring!) when revving, so that is an improvement.
I suspect that I need to dismantle the one carb and find out why it isn't closing up completely at idle... Could this also be the cause of the backfires?

[injunmort]

what is the condition of the bush of the distributor shaft? if the bush is worn, you will have ign.problems as well. it sounds as if it is ign. because you gained improvement by tuning up.